Automatic devices of a power system, e.g. a relay protection device and a stability control device, need to measure a frequency of the power system accurately. An accuracy and speed of the frequency measurement is critical to reliability of the automatic device such as a under frequency load shedding device. In the past, the automatic device issues false action demands sometimes because the frequency measurement is not accurate, which results in unnecessary economic loss.
Conventional frequency measurement technologies mainly include a zero-crossing detection technique and a phase lock technique based on Discrete Fourier Transform (DFT). The accuracy of the zero-crossing technique is readily affected by transient process of the power system such as noises, harmonic waves and so on.
FIG. 1 is a circuit diagram of a frequency measurement device in prior art.
From FIG. 1, it can be known that this technique adopts the phase lock technique based on Discrete Fourier Transform DFT in the frequency measurement, which can filter noises and integral harmonic wave interference in some degree, but such a method adopts the single cycle DFT, which can not filter non-integral harmonic waves (e.g. fractional frequency harmonic interference), drastic change of voltage amplitudes and abrupt change of phase angles effectively. Although a filtering function of some degree can be achieved by using multi-cycle DFT simply, thereby reducing disturbance amplitudes, an overall lasting time of the disturbance is longer than that of the single-cycle DFT instead. Thus, a reliability of the frequency measurement can not be improved efficiently.
Furthermore, it can be known from FIG. 1 that a same control factor G is used in a sampling frequency operation and in a frequency measurement in a discrete phase lock loop for tracking frequencies. The control factor G, such as a proportion control factor for stabilizing time and overshoot, is used to adjust response characteristics. If the control factor G is equal to 1, a frequency stability error will be decreased, but a frequency response will be changed dramatically. If the control factor G is less than 1, i.e. under compensation, the frequency stability error will be increased, but the frequency response will be smooth. Because a sampling frequency and the frequency measurement will be both affected by the same control factor, it is difficult to satisfy both the accuracy of the frequency measurement and the smooth frequency response of the sampling frequency at the same time.
Therefore, in the past, when disturbance occurs in the power system, resulting that voltage amplitudes are fluctuated and phase angles are abruptly changed, or when there is fractional frequency harmonic interference, the frequency measurement value is always not accurate.
Therefore, there is need to develop a new frequency measurement technique which can improve the accuracy of the frequency measurement as much as possible in a case where disturbance occurs in the power system while the requirement for the frequency response speed is satisfied.